Laser devices are now well known, and it is likewise well known to provide a mirror at one, or both, of the opposite ends of the laser tube. It is likewise now well known that such a mirror may be formed by a substrate having a reflective coating thereon, which coating can be formed by applying a plurality of layers of dielectric material to form the reflective coating.
The development of thin-film dielectric coating technology has advanced enormously since the inception of the laser, and, in essence, the laser became practical only after mirrors of very high reflectivity could be manufactured. The layers of film forming the reflective coating are normally vacuum deposited onto polished glass mirror blanks as alternating layers of high and low refractive index materials (usually refractory oxides) designed to generate enhanced reflectance at the selected laser wavelength via constructive interference. In general, the higher the spectral reflectance and the lower all other losses (i.e. diffused scatter, transmission, and absorption), the higher the mirror quality.
It is likewise now well known that heat can be applied to a lens arrangement or the like, including application of heat to a coating on the lens, but such heating has commonly heretofore been utilized to prevent fogging and the like (see, for example, U.S. Pat. Nos. 3,495,259, 4,355,861, 1,791,254, 2,442,913 and 2,376,710).